Last week I interviewed at OU-COM and my first interviewer was a D.O. named Gottfried. He asked me the obvious "Tell me about yourself question", but after describing my experiences, interests, etc, I got the impression that I was leaving something out. He kept grilling me and I eventually realized that what he really wanted was for me to describe my underlying motivation for wanting to practice medicine. Incidentally, I came to some real revelations during this interview. After I gave him the answer he wanted, his demeanor changed and the grilling seemed to stop.
In essence, I think this was the only question he wanted answered. He phrased it specifically after the interview as "What drives you?" He wasn't just talking about medicine, though. He wanted to really understand my underlying principles and beliefs and was willing to help me work them out of my head. So, my suggestion is to think about "what drives you" - both toward medicine and in general. It worked for me, apparently. Dr. Gottfried was the chair of my admissions committee, so he had the most influence on the larger commitee's decision. I must have convinced him because I had an acceptance letter two days later.
--Good luck